Some industrial or commercial locations include conditions, or have the potential to generate conditions, that may be detrimental to a telephone system, low voltage network, or other signal or data communication network. By way of example, a telephone system transmits supervisory signals and analog audio signals which transmit the voice or voice band communications. The supervisory signals include an off-hook signal, indicating that the telephone unit is in use; a ring signal, which causes the bell to sound; and a dial pulse signal, which provides a string of pulses representing the dialed digits of the telephone number. The supervisory signals are low amperage and typically less than one hundred volts. The audio signals are an electric analog signal representing the speech energy generated by the user of the telephone. Alternatively, the audio signal may be voice band data communication, as generated by a modem unit. These communications are typically in the low frequency range of 300 - 3000 Hz and have a maximum amplitude of about 1 volt.
To prevent these sensitive communications from being disrupted, it is often necessary to isolate the telephone line from other electrical or electromagnetic influences. For example, ground potential rise can be caused in an electric power substation by an electrical system short or fault, or a lightening strike that passes a large surge of current to ground. This surge raises the voltage difference between the incoming telephone line ground and the substation earth ground, causing possible flash over, damage to telephone network equipment, and a compromise or loss of any supervisory or audio signals. In the past, isolation equipment has been used in an attempt to protect the telephone line, but the equipment is cumbersome, expensive, and has not been entirely successful, requiring expensive redundant systems or duplication of equipment.
Optical links have also been used in electrical systems to provide electrical isolation. Electrical signals are converted into optical signals and transmitted over fiber optic cable, which is a highly conductive optical link. Typical fiber optic cable has an electrical isolation exceeding one kilovolt per foot, yet its signal attenuation is less than 0.005 dB per meter, with a bandwidth exceeding 100 MHz. These qualities make fiber optic systems superior to traditional telephone isolation equipment because the only link is a cable having a high electrical impedence, yet it efficiently carries a clean signal in the voice band range.
Other fiber optic systems have been used to transmit telephone communications, but they are not compatible with standard telephone supervisory and audio signals, such as those available at an RJ11 or other approved telephone connection. Such systems do not communicate both supervisory and audio signals, they do not reconvert the fiber optic signals to a standard subscriber electrical interface, they are not powered by the telephone line, and do not transmit the signals on a real time basis over dedicated lines with optical signals that are truly representative of the corresponding electrical signals. Thus, these systems could not be freely substituted for standard telephone equipment. Some optical systems transmit analog signals, which results in a weak signal that is subject to attenuation and noisy transmission, as typified by low quality long distance communication. Optical transmission of analog data signals also suffers because the amplitude of the signal varies with the length of the connecting optical cable. Those optical systems utilizing digital transmission techniques are typically time division multiplexed systems, which are extremely complex and may use batch pulse transmission and high speed techniques. Such systems usually do not transmit supervisory signals. Still further, one cannot make a straight metallic-for-optic substitution in the communication link because the power normally transmitted over a metallic conductor is not transmitted by an optical cable.
Accordingly, it is an object of the present invention to provide an optical communication apparatus for bidirectionally transmitting and receiving telephone supervisory and audio signals.
It is a further object of the present invention to provide a transparent link for use in standard electric telephone lines, meaning that it is transparent to the user because it operates like a standard electric telephone line and transparent to the telephone company because it operates like a standard item of approved user telephone equipment.
It is a still further object of the present invention to provide an optical communication apparatus that may be installed as user telephone equipment.
It is a still further object of the present invention to provide an optical communication apparatus that transmits and receives supervisory and audio signals on a real time basis.
It is a still further object of the present invention to provide an optical communication apparatus that responds to telephone supervisory and audio signals moving in either direction.
It is a still further object of the present invention to provide an optical communication apparatus having a telephone line interface unit that is powered by the energy from a standard telephone line.
These and other objects of the present invention are provided by an optical communication apparatus for bidirectionally transmitting and receiving supervisory and audio signals. The apparatus includes a telephone line interface unit that receives electric audio and supervisory signals and converts them to optical signals. It is also includes means for receiving optical signals representing audio and supervisory signals. A data unit receives the optical audio and supervisory signals from the interface unit and converts them to electric audio and supervisory signals, and also includes means for receiving electric audio signals and supervisory signals and converting them to corresponding optical signals. An optical link connects the telephone line interface unit and data unit for bidirectional optical communication. A method of practicing the invention is also included.